Wildlife watch: the great diving beetle

Great diving beetle eating under water. Getty images

The great diving beetle is one of Britain’s largest beetles, with an olive-brown, oval body up to 3cm in length. It’s found in large ponds and other bodies of water, including swimming pools, and is easy to spot as it rises to the surface of the pond to replenish its air supply, which it stores beneath its wings.

Males and females are easy to tell apart: the wing cases of males are smooth while those of females are ribbed, presumably to help males to get a better grip when mating. Both sexes have a yellow margin that runs along the edge of the wing cases and thorax.

Mating takes place in early spring, with eggs laid individually among pond plants. With sufficient food the larvae grow quickly and will leave the pond by late summer to burrow into damp soil around the pond to pupate. Adult beetles emerge from their pupae in mid-autumn but remain in the soil until spring, where they return to the water to mate. They can live for up to three years.

Great diving beetle larvae are yellow-brown, growing to about five centimetres in length, and look similar to rove beetles swimming through the water. Both the adults and larvae are voracious predators and eat other aquatic insects, tadpoles and small fish.

How to help great diving beetle

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The best thing to do for the great diving beetle is to dig a pond and keep it topped up in summer. This will enable them to complete their lifecycle and won’t force them to search for alternative sources of water, which may be polluted or otherwise degraded. Topping up your pond with stored rainwater is best, but if you need to use tap water, leave it in a water butt or watering can for 24 hours to ensure any harmful chemicals evaporate before you add it to the pond.

Great diving beetles and their larvae eat other aquatic invertebrates and larvae, so grow a variety of submerged, floating and emergent plants to provide the best habitats for them. A pond with gentle, sloping sides and beach areas is perfect for frogs, which lay masses of spawn in spring. It might seem odd to encourage one type of species just to be eaten by another, but that’s the food chain, and the better the food chain you can provide in your garden, the better the ecosystem and the better the habitat generally, for all wildlife.

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